The invention relates to an f.theta. single lens.
Recently, a recording system utilizing a laser beam which is intensity-modulated in accordance with a picture signal corresponding to an image pattern to be recorded in order to scan a record medium optically to achieve a recording of an image, finds practical applications as a printer or a receiving unit of a facsimile system.
The laser beam which is intensity-modulated in accordance with the picture signal is converted into a substantially collimated beam, which then impinges upon deflection means such as a rotating multi-facet mirror so as to be deflected with a substantially constant angular speed. The deflected laser beam is then incident on an f.theta., which then focuses it into the recording surface of a photosensitive record medium, thus optically scanning the recording surface in a manner corresponding to the deflection. The f.theta. lens provides a correction for the deflection of the laser beam which takes place at an equal angular speed so that the focusing spot moves at a constant speed across the recording surface. Specifically, when the laser beam impinges on the entrance pupil of the f.theta. lens having a focal length of f, with an angle of .theta. with respect to the optical axis thereof, the beam will be focused onto the recording surface at a point which is displaced by a distance of f.theta. from the optical axis, the designation "f.theta. lens" beging derived from such fact.
In the prior art arrangement, an f.theta. lens comprises a plurality of lenses. However, if the f.theta. lens is formed by a plurality of lenses, the overall length of the f.theta. lens increases, and a high accuracy is required in the combination of various element lenses.
It will be appreciated that the beam impinging on the f.theta. lens has a relatively small diameter and that the beam is deflected only in a given plane. Accordingly, the overall size of the f.theta. lens can be minimized by forming the lens assembly into strip form. However, where a plurality of lenses are used to form the f.theta. lens, the accuracy requirement imposed on the combination or individual lenses presents a great difficulty in forming the strip configuration.
To cope with this problem, an attempt has been made to utilize a single lens for the f.theta. lens. In fact, Japanese Laid-Open Patent Application No. 079,644/1979 and No. 007,727/1980 propose f.theta. single lenses.
In the arrangement proposed in Japanese Laid-Open Patent Application No. 079,644/1979, an incident laser beam has a deflection plane which forms an angle with the optical axis of the f.theta. lens because of the manner of use, and hence there results in a difficulty that a curvilinear locus is traced on the recording surface by the focusing spot of the laser beam. In the arrangement of Japanese Laid-Open Patent Application No. 007,727/1980, the lens represents a plano-convex lens, which presents a difficulty in connection with the curvature of the image surface.